The present invention relates generally to ultrasound imaging, and more specifically to imaging techniques in a post-storage mode.
The following references are incorporated by reference:                U.S. Pat. No. 4,265,126: Papadofrangakis et al. (1981) “Measurement of true blood velocity by an ultrasound system.”        U.S. Pat. No. 4,604,697: Luthra et al. (1986) “Body imaging using vectorial addition of acoustic reflection to achieve effect of scanning beam continuously focused in range.”        U.S. Pat. No. 5,365,929: Peterson (1994) “Multiple sample volume spectral Doppler.”        U.S. Pat. No. 5,409,010: Beach et al. (1995) “Vector Doppler medical devices for velocity studies.”        U.S. Pat. No. 5,415,173: Miwa et al. (1995) “Ultrasound diagnosis system.”        U.S. Pat. No. 5,690,111: Tsujino (1995) “Ultrasound diagnostic apparatus.”        U.S. Pat. No. 6,221,020: Lysyansky et al. (2001) “System and method for providing variable ultrasound analyses in a post-storage mode.”        U.S. Pat. No. 6,263,094: Rosich et al. (2001) “Acoustic data acquisition/playback system and method.”        U.S. Pat. No. 6,450,959: Mo et al. (2002) “Ultrasound B-mode and Doppler flow imaging.”        U.S. Pat. No. 6,679,847: Robinson et al. (2004) “Synthetically focused ultrasonic diagnostic imaging system for tissue and flow imaging.”        U.S. Pat. No. 6,860,854: Robinson et al. (2005) “Synthetically focused ultrasonic diagnostic imaging system for tissue and flow imaging.”        U.S. Pat. No. 6,926,671: Azuma et al. (2005) “Ultrasonic imaging apparatus and method.”        U.S. patent application Ser. No. 11/492,471, filed Jul. 24, 2006: Napolitano et al. “Continuous Transmit Focusing Method And Apparatus For Ultrasound Imaging System.”        Tortoli et al. (1985) “Velocity profile reconstruction using ultrafast spectral analysis of Doppler ultrasound.” IEEE Transactions Sonics and Ultrasonics, vol. SU-32, pp. 555-561.        Nitzpon et al (1995) “New pulsed wave Doppler u/s system to measure blood velocities beyond the Nyquist limit.” IEEE Transactions Ultrason., Ferroelec. and Freq. Cntrl., vol. UFFC-42, pp. 265-279.        
In ultrasonic B-mode imaging, a two-dimensional (2D) image of tissue is created in which the echo intensity is mapped to pixel brightness. For continuous wave (CW) measurement of blood flow in the heart and vessels, the frequency shifts of backscattered ultrasound waves are used to estimate blood velocities. For pulsed wave (PW) measurement of blood flow, the phase shifts of backscattered ultrasound waves from a number of transmit excitations are used for flow estimation. In 2D color flow imaging and timeline color-M mode, the mean phase shift, which is proportional to the motion-induced Doppler frequency shift, is displayed using different colors that represent different flow speed. In spectral Doppler mode, the power spectrum of Doppler signals are computed and displayed as velocity-time waveforms. Contrast agents may be employed with any of the imaging modes to further enhance the signal-to-noise or signal-to-clutter ratio.
A typical ultrasound imaging system will include the following main subsystems: a transmitter, a receiver, a receive focusing unit, a cine memory buffer, an image processor/display controller, a display unit, and a master controller.
FIG. 1 is a block diagram of a conventional ultrasound system for which the receive array focusing unit is referred to as a beamformer, and image formation is performed on a scan-line-by-scan-line basis. System control is centered in the master controller, which accepts operator inputs through an operator interface and in turn controls the various subsystems. For each scan line, the transmitter generates a radio-frequency (RF) excitation voltage pulse waveform and applies it with appropriate timing across the transmit aperture (defined by a sub-array of active elements) to generate a focused acoustic beam along the scan line. RF echoes received by the receive aperture of the transducer are amplified and filtered by the receiver, and then fed into the beamformer, whose function is to perform dynamic receive focusing; i.e., to re-align the RF signals that originate from the same locations along various scan lines.
To reduce the data sampling rate requirements, the RF data is often converted (not shown) into baseband I/Q data before or after the beamformer. In the ZONARE Z.one™ system, a synthetic array focusing approach such as shown in U.S. patent application Ser. No. 11/492,471, filed Jul. 24, 2006 for “Continuous Transmit Focusing Method And Apparatus For Ultrasound Imaging System” (David J. Napolitano et al.) is used for 2D imaging. Specifically, a complete set of echo data obtained from a sequence of transmit-receive cycles is accumulated in digital memory first, and then combined coherently in the receive focusing unit to produce the effect of having continuously focused transmit and receive beams throughout the image field. Other methods of synthetic focusing have also been described in U.S. Pat. Nos. 4,604,697, 6,679,847, and 6,860,854, for both B-mode and flow imaging.
The image processor performs the processing specific to the active imaging mode(s) including 2D scan conversion that transforms the image data from an acoustic line grid to an X-Y pixel image for display. For Spectral Doppler mode, the image processor performs wall-filtering followed by spectral analysis of Doppler-shifted signal samples using typically a sliding FFT-window. It is also responsible for generating the stereo audio signal output corresponding to forward and reverse flow signals. In cooperation with the master controller, the image processor also formats images from two or more active imaging modes, including display annotation, graphics overlays and replay of cine loops and recorded timeline data.
The cine buffer provides resident digital image storage for single image or multiple image loop review, and acts as a buffer for transfer of images to digital archival devices. On most systems, the video images at the end of the data processing path can be stored to the cine memory. In state-of-the-art systems, amplitude-detected, beamformed data may also be stored 22 in cine memory. For spectral Doppler, some machines store the wall-filtered, baseband Doppler I/Q data for the user-selected range gate in cine memory, and the ZONARE z.one system stores the pre-wall-filtered data.
U.S. Pat. No. 6,263,094 describes a system wherein raw or partially processed data acquired early in the pipeline (pre-beamformed, post-beamformed, pre-video, etc.) is stored in non-volatile memory, and can be introduced into a signal processing system from memory at least at the rate that it was acquired, to produce a real time image that can be modified by the reviewer by further processing, if desired. The strict requirement of introducing the acoustic data at least at that particular rate is to ensure the real time playback is “as if the insonified target was the source of the acoustic data, not a storage device.” [4:30-31].
For motion imaging, the cost of storing Doppler data from earlier points in the receive data path is greater cine memory capacity (or shorter image loops for the same memory size) and increased post-storage processing load (and response time). The benefits are encapsulated by the concept of a “virtual patient;” i.e., the sonographer can focus on setting the probe over the region of interest, and storing the raw (i.e., unprocessed) Doppler data first. Then, during cine review, the operator can take time to re-adjust the image processing parameters based on the same data set (the “virtual patient”) without keeping the patient around longer than needed. Considering the fact that sonographers need to manage many front-panel system controls with one hand, hold the probe with the other hand, and deal with the patient's body movements, the workflow and ergonomic advantages of such post-storage data processing capabilities are clear.
U.S. Pat. No. 6,221,020 describes a system wherein beamformed (I/Q or RF) data from a region of interest is accumulated 24 in cine memory during a storage period. As illustrated in FIG. 2, the receive beamforming refers to a weighted delay-and-sum operation, and the amplitude weighting across receive aperture is referred to as “apodization,” which is important for suppressing the sidelobes of the receive beam profile. This method enables, during post-storage playback operation, any known signal processing and system control which have conventionally been carried out in real-time during the scanning session. For example, in a playback mode, the user may select any scan line and Doppler gate location and width within the region of interest for spectral Doppler or for color M-mode analysis. However, a limitation of this prior art is that the post-storage processing capabilities are restricted to conventional methods based on ultrasound data that is already beamformed.
Due to patient-to-patient variations and different anatomical scenarios in different clinical applications, a number of important system controls actually need to be effected before or during the beamforming process. For example, it is well known that the sound speed parameter used for array focusing operations can vary with different organ types and from patient to patient. Any sound speed optimization must be performed during the receive array focusing or reconstruction process. In particular, the ZONARE z.one system supports channel domain processing that first stores all the raw transducer element data in a channel domain memory, and then allows digital signal processor units to access the channel data multiple times in order to support an iterative algorithm that optimizes the array focusing sound speed. It is important to note that this kind of adaptive processing may take up to a few seconds to complete while the rest of the signal pipeline is paused; i.e., the channel data is often read out by the signal processors multiple times and at a rate that may be lower than when it was acquired.
In spectral Doppler mode, the receive aperture, which determines the receive F-number (defined by the ratio of focal range to aperture size), directly controls the degree of receive focusing. A lower F-number means stronger focusing or a tighter beam width. For small vessels or weak signals from deep lying vessels, it is advantageous to maximize the receive aperture size for best sensitivity/penetration. On the other hand, it is well known that because different elements of the active aperture form different angles with the flow direction, a large aperture (low F-number) can give rise to an increased velocity over-estimation errors—an effect referred to as geometrical spectral broadening.
U.S. Pat. No. 6,679,847 describes the synthetic focus system that stores raw channel data and applies different beamforming delay curves to track blood motion. This invention is also aimed at providing flexible motion analyses and display methods, but it is fundamentally a pure synthetic focus system that utilizes single or small groups of elements for transmit (with known sensitivity challenges associated with limited acoustic power outputs) and requires an entire set of uncombined channel data to be stored for each image frame.